Abstract

Among diatomic molecular halogen solids, high pressurestructures of solid chlorine (Cl2) remain elusive and least studied. We here report first-principles structural search on solid Cl2 at high pressures through our developed particle-swarm optimization algorithm. We successfully reproduced the known molecular Cmca phase (phase I) at low pressure and found that it remains stable up to a high pressure 142 GPa. At 150 GPa, our structural searches identified several energetically competitive, structurally similar, and modulated structures. Analysis of the structural results and their similarity with those in solid Br2 and I2, it was suggested that solid Cl2 adopts an incommensurate modulated structure with a modulation wave close to 2/7 in a narrow pressure range 142–157 GPa. Eventually, our simulations at >157 GPa were able to predict the molecular dissociation of solid Cl2 into monatomic phases having body centered orthorhombic (bco) and face-centered cubic (fcc) structures, respectively. One unique monatomic structural feature of solid Cl2 is the absence of intermediate body centered tetragonal (bct) structure during the bco → fcc transition, which however has been observed or theoretically predicted in solid Br2 and I2. Electron-phonon coupling calculations revealed that solid Cl2 becomes superconductors within bco and fcc phases possessing a highest superconducting temperature of 13.03 K at 380 GPa. We further probed the molecular Cmca → incommensurate phase transition mechanism and found that the softening of the Ag vibrational (rotational) Raman mode in the Cmca phase might be the driving force to initiate the transition.

Received 27 March 2012Accepted 19 July 2012Published online 08 August 2012

Acknowledgments:

The authors acknowledge the High Performance Computing Center of Jilin University for supercomputer time and for funding from the China 973 Program under Grant No. 2011CB808200, the National Natural Science Foundation of China under Grant Nos. 11025418, 91022029, 11104104, and 11164020, the research fund of Key Laboratory of Surface Physics and Chemistry (No. SPC201103), and the Changjiang Scholar and Innovative Research Team in University (No. IRT1132).